Progressively stricter emission standards have resulted in the development of improved three-way catalysts by utilizing improved formulations and layered structures. Improvements in thermal stability and durability have also led to catalysts that can survive accelerated aging tests at temperatures of up to 1100° C.
One approach to improving catalyst performance has been to increase the quantity of platinum group metal (PGM) in the catalyst formulations. However, this in turn means an unacceptable increase in costs. Several methods have been used to overcome this problem. A Pd-only TWC catalyst has been developed (1) which uses palladium rather than platinum, to reduce costs. A further improvement to this concept reduces the amount of Pd by incorporating the Pd into a perovskite oxide (2). Palladium has a stronger tendency than Pt to undergo grain growth and hence becomes less active during high temperature operation. Incorporating the Pd into the perovskite structure during the oxidizing cycle of engine operation and in turn reducing the Pd to metal on the surface of the catalyst during the reducing cycle avoids excessive grain growth of the Pd. An additional advantage of using perovskite oxides is their high oxygen storage capacity (OSC), an essential property for good TWC performance. The OSC of certain perovskite has been shown to be even higher than CeO2, the material most commonly used in TWC as the OSC component.
Perovskite oxides have been investigated for catalytic oxidation and reduction reactions associated with the control of automotive exhaust emissions since the early 1970's [6-15]. Perovskite catalysts incorporating different amounts of PGM have been disclosed in U.S. Pat. No. 3,865,923 by Stephens, U.S. Pat. Nos. 3,884,837 and 4,001,371 by Remeika et al., U.S. Pat. Nos. 3,897,367; 4,049,583 and 4,126,580 by Lauder, U.S. Pat. No. 4,107,163 by Donohue, U.S. Pat. No. 4,127,510 by Harrison, U.S. Pat. No. 4,151,123 by McCann, III, U.S. Pat. No. 4,921,829 by Ozawa et al., U.S. Pat. No. 5,318,937 by Jovanovic et al., U.S. Pat. No. 5,380,692 by Nakatsuji et al. and U.S. Pat. Nos. 5,939,354; 5,977,017; 6,352,955; 6,372,686 and 6,351,425 B2 by Golden.
Recent studies have shown that catalysts designed to meet low emission vehicle (LEV) and ultra low emission vehicle (ULEV)-type standards and contain reduced levels of PGM are significantly inhibited by sulfur in the fulel (3). A variety of factors influence the loss of catalytic performance due to sulfur. These factors include the level of sulfur in the fuel, the catalyst design, catalyst location and catalyst composition. The individual components of a TWC catalyst are the catalytic PGM, the OSC component and the support. All three of these components can be affected by sulfur.
In particular, efforts are being made to minimize the effects of sulfur on the PGM and the OSC components of the TWC materials. Palladium although lower cost, is more susceptible to sulfur poisoning than Pt and Rh. Research has shown (4) that metal-metal bonds can significantly reduce the affinity of these metals for SO2. For example Pd/Rh is more tolerant to the presence of sulfur-containing molecules in the fuel than pure Pd catalysts.
SO2 interacts with TWCs that have a ceria-containing component and it is the poisoning of the ceria that appears to be the primary problem associated with sulfur inhibition of these catalysts (5). To improve the sulfur tolerance and increase thermal stability, ceria is commonly mixed in solid solution with zirconia. A combination of ceria-zirconia can also enhance OSC at high operating temperatures.
While perovskite oxides can beneficially incorporate PGMs into their structure many perovskite compositions are susceptible to sulfur poisoning. Perovskite compositions typically incorporate transition metals such as copper, cobalt and manganese into the B site of the ABO3 perovskite formula. Many of these transition metals form stable sulfates with SO2.
U.S. Pat. No. 6,569,803 by Takeuchi claim a catalyst for purifying exhaust gas comprising a perovskite of the general formula ABO3 where the major proportion of the B site ion always includes an element from group consisting Mn, Co and Fe. However, no evaluation was made of resistance of these catalysts to SO2.
In a comprehensive review of perovskites in catalysis L. G. Tejuca, J. L. J. Fierro and J. M. D. Tascon (16) report on extensive catalysis tests of perovskites using Mn, Co and Fe on the B site of the perovskite and concluded, “These results show that poisoning effects of SO2 on these perovskites takes place through adsorption of this molecule on the B sites . . . . SO2 may also interact with cations in position A, but this process does not result in deactivation of the catalyst”. They go on to conclude that, “Although some progress has been attained in the preparation of highly active perovskites for CO and hydrocarbon oxidation and NO reduction by incorporation of noble metals (Pt and Ru) into the structure, the problem of SO2 poisoning remains basically unsolved.
Further complications when incorporating perovskite oxides in TWC formulations included, maintaining phase purity of complex perovskite, achieving thermal stability and producing materials with the high surface areas necessary for good TWC performance at high operating temperatures.
It is an object of the present invention to provide a catalyst containing a perovskite component. The catalyst may be of high surface area, thermally stable, have reduced PGM component and show improved resistance to sulfur inhibition.